Also the best way to feed the extruder is from the bottom, maybe now it doesn't look so fancy, but it's working :)You can see the last results on the images bellow:

updates:

I have been using ninjaflex for a while and trying more different settings, for models that have large surfaces and the volume is made with an interior grid, I have to slow down the speed with the pugin "Tweak at Z" at least 3 layers otherwise you will be able to see holes that weren't covered totally.After to identify the layer (layer view) when the model goes from the grid to a solid surface I slow down the speed to 50%. The faster speed that I have tried and works pretty well is 40mm/s, so it will be 20mm/s to have a smooth final surface.You can appreciate the difference in the image below:

left: same speed the whole processright: slowing down the speed from layer 25 to 28